In known tank-venting systems, fuel vapors develop in the tank and are intermediately stored in an active charcoal filter and are then supplied via a tank-venting valve to the intake pipe of the engine. U.S. Pat. No. 4,318,383 describes a method wherein the tank-venting valve is only then opened when a certain quantity of vaporous fuel is present in the system. The system described in this patent provides that the temperature of the fuel or the overpressure in the tank region is a measure for this quantity. Statutory requirements are provided for monitoring these emission-relevant systems.
U.S. Pat. No. 5,193,512 discloses a tank-venting system which is equipped with a shutoff valve in the venting line of the active charcoal filter. Overpressures as well as underpressures can be adjusted in the tank-venting system with a deliberate opening and closing of this shutoff valve in dependence upon the opening state of the tank-venting valve. These pressure changes are detected by a difference pressure sensor on the tank and the evaluation of these pressure changes makes it possible to provide a statement as to the operability of the tank-venting system. The active charcoal filter is scavenged with fresh air (regeneration) with an opened shutoff valve and a clocked opening of the tank-venting valve. However, the following problem can occur when scavenging the charcoal filter under these conditions. The flow resistance of the active charcoal in the active charcoal filter causes a pressure drop to occur at the active charcoal filter. This pressure drop becomes that much greater the greater the flow resistance of the active charcoal is at a pregiven intake pressure which is determined by the underpressure in the intake pipe, the opening cross section of the tank-venting valve and the conduit geometry. If this resistance increases, for example because of deterioration, then the absolute pressure at the intake end of the active charcoal filter and therefore at the fuel tank drops. This causes, on the one hand, that the tank itself can become damaged and, on the other hand, a low absolute pressure in the tank is unwanted because it causes the fuel to vaporize. It is known to avoid these disadvantages by providing an additional flow resistor and to provide the same, for example, in the form of a flow throttle in the connection of the active charcoal filter to the intake pipe.